Thin Yba[SUB2]Cu[SUB3]O[SUB7-δ/La[SUB0.67]Ca[SUB0.33]MnO[SUB3] (YBCO/LCMO) films were grown on SrTiO[SUB3](STO)substrates by magnetron sputtering technique. The microstructures of the bilayers were characterized and a standard four-probe technique was applied to measure the resistivity of the samples. The interdiffusions at the YBCO/LCMO and LCMO/STO interfaces formed two transient layers with the thickness of about 3 and 2 nm, respectively. All the bilayers were well textured along the c axis. At low temperature, the superconductivity can only be observed when the thickness of YBCO is more than 25 nm. When the thickness of YBCO is less than 8 nm, the bilayers show only ferromagnetism. The superconductivity and ferromagnetism perhaps coexist in the bilayer with the YBCO thickness of 12.5 nm. These interesting properties are related to the interaction between spin polarized electrons in the manganites and the cooper pairs in the cuprates. © 2003 American Institute of Physics.published_or_final_versio

Du, J

Gao, J

Hu, A

Jia, QJ

Jiang, SS

Liu, JS

Tan, WS

Wang, J

Wu, XS

Wu, ZH

Zheng, WL

Journal of Applied Physics

HKU Scholars Hub

Title Microstructures and resistivity of cuprate/manganite bilayerdeposited on SrTiO3 substrateAuthor(s) Tan, WS; Wu, XS; Du, J; Liu, JS; Hu, A; Jiang, SS; Wang, J; Wu,ZH; Zheng, WL; Jia, QJ; Gao, JCitation Journal of Applied Physics, 2003, v. 93 n. 10, p. 8215-8217Issued Date 2003URL http://hdl.handle.net/10722/42481Rights Creative Commons: Attribution 3.0 Hong Kong LicenseJOURNAL OF APPLIED PHYSICS VOLUME 93, NUMBER 10 15 MAY 2003Microstructures and resistivity of cuprateÕmanganite bilayer depositedon SrTiO3 substrateW. S. Tan,a) X. S. Wu, J. Du, J. S. Liu, A. Hu, and S. S. JiangDepartment of Physics, National Laboratory of Solid State Microstructures, Nanjing University,Nanjing 210093, ChinaJ. Wang, Z. H. Wu, W. L. Zheng, and Q. J. JiaInstitute of High Energy Physics, Chinese Academy of Science, Beijing 100039, ChinaJ. GaoDepartment of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong, China~Presented on 14 November 2002!Thin YBa2Cu3O7-d /La0.67Ca0.33MnO3 ~YBCO/LCMO! films were grown on SrTiO3(STO)substrates by magnetron sputtering technique. The microstructures of the bilayers werecharacterized and a standard four-probe technique was applied to measure the resistivity of thesamples. The interdiffusions at the YBCO/LCMO and LCMO/STO interfaces formed two transientlayers with the thickness of about 3 and 2 nm, respectively. All the bilayers were well textured alongthe c axis. At low temperature, the superconductivity can only be observed when the thickness ofYBCO is more than 25 nm. When the thickness of YBCO is less than 8 nm, the bilayers show onlyferromagnetism. The superconductivity and ferromagnetism perhaps coexist in the bilayer with theYBCO thickness of 12.5 nm. These interesting properties are related to the interaction between spinpolarized electrons in the manganites and the cooper pairs in the cuprates. © 2003 AmericanInstitute of Physics. @DOI: 10.1063/1.1541653#I. INTRODUCTIONThe classical proximity effect between the conventionalsuperconductor ~S! and metallic ferromagnet ~F! thin layershas been studied in many systems theoretically andexperimentally.1–6 It is now believed that the heterostructure,combined high Tc superconductor ~HTS! with colossal mag-netoresistance ~CMR! oxide, has potential application in fastdevices with high gain, due to the almost full spin polariza-tion of CMR oxides, together with the fast relaxation timesand low carrier density of HTS. In most cases, perfect crys-talline structure will play an important role in ensuring theproperties of devices. Fortunately, the similarity in the struc-ture of high Tc cuprate and CMR manganite makes it pos-sible to prepare high quality heterostructures. Therefore,many studies have been performed on HTS/CMR F/Sheterostructures, especially on cuprate/manganite hetero-structures.7–11 Most of these studies are devoted to the super-conductivity and magnetism of cuprate/manganite multilay-ers and superlattices. It is generally believed that the super-conductivity is suppressed by spin-polarized carriers injectedinto superconductor.12,13 In addition, it is significant to pointout that in Ref. 13 the value of self-injection length of spin-polarized carriers ~77 K! was estimated to be ; 90 nm in aLa0.7Ca0.3MnO3 /YBa2Cu3O72d bilayer from the measure-ment of critical current density Jc .Although many studies have been performed in cuprate/manganite multilayers and superlattices as mentioned above,there is little work on the relationship between microstruc-a!Author to whom correspondence should be addressed; electronic mail:ssjiang@nju.edu.cn8210021-8979/2003/93(10)/8215/3/$20.00tures and transport properties of heteroepitaxial bilayers. Inthis article we present the microstructure and transport prop-erties of YBa2Cu3O7-d /La0.67Ca0.33MnO3 ~YBCO/LCMO!bilayers.II. EXPERIMENTThin YBCO/LCMO films were grown on ~100!SrTiO3(STO) substrates by the off-axis rf magnetron sput-tering technique. The total sputtering pressure of 25%oxygen–75% argon mixture was kept at 10–20 Pa. The de-positing temperature was 750–770 °C for LCMO and 730–750 °C for YBCO, respectively. The growth rate for LCMOand YBCO was 0.6–0.7 and 0.9–1.0 nm/min, respectively.All the samples were postannealed for 0.5 h at 500 °C in 1atm of pure oxygen. In YBCO/LCMO/STO heterostructuresstudied here, the LCMO thickness was fixed at 30 nm andthe YBCO thickness t for different samples are listed inTable I.The microstructures of samples were characterized bythe high resolution x-ray diffraction, grazing incident x-rayreflectivity ~GIXR!, Auger electron spectroscopy ~AES!, andTABLE I. The YBCO thickness of YBCO/LCMO(30 nm)/STO(t) hetero-structure.Sample t (nm)A 50B 25C 12.5D 8E 45 © 2003 American Institute of Physics8216 J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Tan et al.atomic force microscopy ~AFM!. A dc four-probe methodwas used to measure the resistivity of the samples withoutmagnetic field applied.III. RESULTS AND DISCUSSIONFigure 1 shows high angle x-ray diffraction pattern forsamples, in which only (00L)-type diffraction peaks can befound. This means that all the samples were well c axisgrown. From the (00L) peak position we can learn that withincreasing the YBCO thickness the lattice parameter c ofYBCO layer decreases from 1.1744 to 1.1680 nm, which isalmost the value of that for YBCO bulk material. The in-plane average parameter a will increase with the increase ofthe thickness of YBCO. This means that the lattice of YBCOlayer is partially strained ~expanded! due to the latticemismatch and finally total relaxation of strain occurs forsample A.To investigate the microstructures at the surface and in-terface of film, GIXR and AFM were applied. Figure 2shows the measured and simulated GIXR curves for samplesFIG. 1. High angle X-ray diffraction patterns for samples. The capitalizedletters S, L, and Y refer to STO, LCMO, and YBCO, respectively. Thenumbers in parentheses is Miller indices.FIG. 2. The measured and simulated grazing x-ray reflectivity profiles forsamples C and E.C and E. Here the theoretical simulation is based onFresnel’s law.14 The corresponding simulation parameters arelisted in Table II, from which one can see that there is adifference between the nominal and real thickness of thesublayers and transient layer existing at the YBCO/LCMOand LCMO/STO interfaces, with the thicknesses of about 3and 2 nm, respectively. The results of AES, shown in Fig. 3,confirmed that the transient layer is caused by the interdiffu-sion at the interface. In addition, the surface root-mean-square ~rms! roughness for samples C and E is, respectively,0.45 and 0.74 nm, which is consistent with the observationfrom AFM within experimental error. From the above re-sults, we can conclude that YBCO film, deposited on aLCMO template layer, is partially strained due to the latticemismatch and the surface is flat in spite of the existence of atransient layer.Figure 4 presents the temperature dependence of sampleresistivity without magnetic field applied. Samples A and Bare superconducting with the critical temperature Tc0 ~mid-point, Tc) at 83 K. The critical current density at 77 K isestimated to be larger than 0.53105 A/cm2. The supercon-ductivity is obviously depressed while comparing with thoseFIG. 3. The depth dependence of element Cu, La, and Ti composition insample C.FIG. 4. The temperature dependence of resistance for samples without mag-netic field applied.8217J. Appl. Phys., Vol. 93, No. 10, Parts 2 & 3, 15 May 2003 Tan et al.TABLE II. The simulation results of the grazing incident x-ray reflectivity profiles.SimulationresultsSimulationlayersSimulationthickness ~62 Å!Mass density~g/cm3!Surface and interfaceroughness of layers~60.5 Å!Sample CSurf. 19 3.43 ssurf54.5YBCO 134 6.27 sYBCO55.0TransientLayer233 6.19 sTrans254.3LCMO 308 6.51 sLCMO54.2TransientLayer122 6.21 sTrans152.1STO – 5.12 sLCMO/STO54.6Sample ESurf. 12 3.98 ssurf57.4YBCO 48 6.23 sYBCO56.8TransientLayer222 6.09 sTrans255.5LCMO 327 6.44 sLCMO55.2TransientLayer116 5.98 sTrans151.7STO – 5.12 sLCMO/STO54.8YBCO films using other nonmagnetic buffers such asNd2CuO4 .15 For samples D and E, there only appears a re-sistivity maximum with TM at about 210 and 250 K, respec-tively. It is well known that in the R – T curve of perovskitemanganite TM corresponds to the metal–insulator transitionand it is a little lower than the ferromagnetic–paramagnetictransition temperature TCurie . As for sample C, the supercon-ductivity is still observed, but Tc0 has dropped to 78 K.Sample C also shows resistance maximum with TM at about170 K at the same time. This means that superconductivityand ferromagnetism perhaps coexist in sample C with theYBCO thickness of 12.5 nm.The transport properties described above clearly implythat superconductivity is competing with ferromagnetism inLCMO/YBCO bilayers. Considering the pair breaking effectenhanced by the spin-polarized carriers injection, the super-conductivity will be suppressed because a certain thicknessof YBCO will become ‘‘effectively’’ normal. Thereby theinjection length is roughly estimated to be less than 25 nm.This value is much less than the results reported in Ref. 13.It is still unclear whether the injection length is related to theLCMO thickness or not. Most interestingly, superconductiv-ity and ferromagnetism coexist but are suppressed by eachother in sample C with the YBCO thickness of 12.5 nm. TheYBCO thickness and thereby the strain state of bilayer isanother factor for the transport properties of YBCO/LCMOheterostructures. As can be seen from Fig. 4, for samples C,D, and E, metal–insulator transition temperature TM isstrongly influenced by the YBCO thickness and thereby thestrain state of bilayers. In addition, the disorder at the inter-face may also be an influence factor that should not be neg-ligible.IV. CONCLUSIONSIn summary, we prepared well ~001!-oriented YBCO/LCMO layered materials by magnetron sputtering. The mi-crostructures and transport properties were investigated. Theresults show that superconductivity and ferromagnetism per-haps coexist in the bilayer with the YBCO thickness of 12.5nm; while in the sample with YBCO thickness less than 8nm, only ferromagnetism was observed, with metal–insulator transition temperature TM strongly influenced bythe strain state of bilayers.ACKNOWLEDGMENTSThis work has been supported by the National NaturalScience Foundation of China, the Provincial Natural ScienceFoundation of Jiangsu, the State Key Project of FundamentalResearch, and the Chinese Ministry of Education.1 Z. Radovic, L. Dobrosavljevic-Grujic, A. I. Buzdin, and J. R. Clem, Phys.Rev. B 38, 2388 ~1988!.2 Z. Radovic, M. Ledvij, L. Dobrosavljevic-Grujic, A. I. Buzdin, and J. R.Clem, Phys. Rev. B 44, 759 ~1988!.3 C. A. R. Sa de Melo, Phys. Rev. Lett. 79, 1933 ~1997!.4 H. K. Wong and J. B. Ketterson, J. Low Temp. Phys. 63, 139 ~1986!.5 C. Strunk, C. Surgers, U. Paschen, and H. V. Lohneysen, Phys. Rev. B 49,4053 ~1994!.6 L. Lazar, K. Westerholt, H. Zabel, L. R. Tagirov, Yu. V. Goryunov, N. N.Garif’yanov, and I. A. Garifullin, Phys. Rev. B 61, 3711 ~2000!, andreferences therein.7 K. Chahara, T. Ohno, M. Kasai, Y. Kanke, and Y. Kozono, Appl. Phys.Lett. 62, 780 ~1993!.8 G. Jakob, V. V. Moshchalkov, and Y. Bruynseraede, Appl. Phys. Lett. 66,2564 ~1995!.9 Y. Inokuma, M. Kanai, and T. Kawai, Jpn. J. Appl. Phys., Part 1 36, 7169~1997!.10 P. Prieto et al., J. Appl. Phys. 89, 8026 ~2001!.11 Z. Sefrioui et al., http://xxx.lanl.gov/cond-mat/0207164.12 V. A. Vas’ko, V. A. Larkin, P. A. Kraus, K. R. Nikolaev, D. E. Grupp, C.A. Nordman, and A. M. Goldman, Phys. Rev. Lett. 78, 1134 ~1997!.13 S. P. Pai, S. Wanchoo, S. C. Purandare, T. Banerjee, P. R. Apte, A. M.Narsale, and R. Pinto, Pramana J. Phys. 58, 1147 ~2002!.14 L. G. Parrat, Phys. Rev. 95, 359 ~1954!.15 J. Gao, W. H. Tang, and T. C. Chui, Physica C 330, 33 ~2000!.

Microstructures and resistivity of cuprate/manganite bilayer deposited on SrTiO3 substrate

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Microstructures and resistivity of cuprate/manganite bilayer deposited on SrTiO3 substrate

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